12 THE PROTOZOA 



smaller free-swimming form which has originated 

 from repeated division of a large form. Secondly, 

 the union of the two is a complete and permanent 

 fusion, the smaller being absorbed into the larger. 



This permanent fusion of a small active cell with 

 a relatively large fixed cell, followed by division of 

 the fused mass, bears a striking analogy to the pro- 

 cess of sexual reproduction occurring in higher 

 animals. 



3. Encystment may occur after conjugation, and apparently 

 as the result of it. The disc becomes retracted ; 

 the cell-body usually separates from its stalk, be- 

 comes spherical, and secretes a cyst around itself; 

 the nucleus enlarges and becomes greatly elongated 

 and then moniliform, and finally breaks up into a 

 number of ' spores.' These are ovoid bodies, each 

 with a circlet of cilia. The cyst ruptures and the 

 spores escape and swim freely, multiplying by fis- 

 sion ; later on each becomes attached by the end 

 surrounded by the cilia and develops a disc at the 

 opposite end, loses its original zone of cilia, and 

 grows up into an ordinary Vorticella. 



This process is not of frequent occurrence. 



D. Action of Eeagents. 



1. Kill some Vorticellce with acetic acid (1 per cent, or 



weaker). Stain with magenta or carmine, and 

 mount in glycerine, or balsam. 



The nucleus will be stained, and its shape and 

 relations can be well studied. 



2. Weak solutions of poisons such as acetic acid, corrosive 



sublimate, etc., cause the animal to retract its disc, 

 and the stalk to gradually coil up into a close spiral, 

 and then break off close to the base of the bell. In 

 the natural condition the cell-body may separate in 

 the same way from the stalk and swim away to a 

 fresh resting-place, when scarcity of food or other 

 adverse circumstance renders it advantageous. 



